In recent years, the development of nonaqueous batteries typified by lithium ion secondary batteries has been energetically performed. Since the nonaqueous batteries contribute to reduction in size and weight of power sources, they are used as power sources for many products such as notebook personal computers, mobile telephones, electric power tools, and electronic communications equipment, and recently have attracted attention also as environmentally friendly power sources for vehicles in which the amount of emission of harmful substances is small, such as electric vehicles and hybrid electric vehicles. However, conventionally known nonaqueous batteries are not necessarily sufficient in terms of output, capacity, and life, and higher output, higher capacity, and longer life are required.
The nonaqueous battery is a secondary battery comprising a positive electrode using a metal oxide or the like as an active material, a negative electrode using a carbon material such as graphite as an active material, and an electrolyte solvent such as carbonates and a flame-retardant ionic liquid, in which charge and discharge of a battery is performed by the movement of ions between the positive electrode and the negative electrode. Generally, the positive electrode is obtained by coating the surface of a positive electrode collector such as aluminum foil with a slurry comprising a metal oxide and a binder, drying the coating, and then cutting the coated positive electrode collector to a suitable size. The negative electrode is obtained by coating the surface of a negative electrode collector such as copper foil with a slurry comprising a carbon material and a binder, drying the coating, and then cutting the coated negative electrode collector to a suitable size. Therefore, each binder has a role of binding active materials to each other and binding an active material to a collector to prevent peeling of active materials from a collector.
As the binder, a polyvinylidene fluoride (PVDF)-based binder using N-methyl pyrrolidone (NMP) of an organic solvent system as a solvent is well known. However, this binder has a low binding property between active materials and between an active material and a collector, and a large amount of binder is required for practical use. As a result, the capacity of the resulting nonaqueous battery is reduced, which is a defect. In addition, since expensive and poisonous NMP is used as a solvent for binders, there is a problem also in the price of end products and the preservation of work environment during the production of a slurry or a collector.
As a method for solving these problems, the development of a water-dispersed binder has been advanced. For example, there is known a styrene-butadiene rubber (SBR)-based aqueous dispersion in which carboxymethyl cellulose (CMC) is used in combination as a thickener. Since this SBR-based dispersion is an aqueous dispersion, it is inexpensive and advantageous from the point of view of the preservation of work environment. Further, since the binding property between active materials and between an active material and a collector is relatively satisfactory, electrodes can be produced by using a smaller amount of the SBR-based dispersion than the amount of the PVDF-based binder. As a result, there is an advantage that higher output and higher capacity of nonaqueous batteries can be achieved. From these advantages, the SBR-based dispersion is widely used as a binder for nonaqueous battery electrodes.
However, also in this binder, the binding property between active materials and between an active material and a collector is not necessarily sufficient, and when an electrode is produced with a small amount of binder, a part of an active material is peeled off in the step of cutting a collector, which is problematic. Further, when the SBR-based binder is used, the resistance value of the resulting nonaqueous battery tends to be high. As a result, there is a problem that higher output and longer life cannot be achieved.
The research to aim at the improvement in performance of the water-dispersed binder typified by SBR has been advanced under such a background. For example, for the purpose of improving the charge-discharge cycle property of an electrode, there is proposed a method involving adding acetylene glycol or a derivative thereof when a water-dispersed binder is mixed with an active material (PTL 1). According to this method, the water-dispersed binder is used in an amount of 2 to 15 parts by mass in terms of solids per 100 parts by mass of the active material, and acetylene glycol or a derivative thereof is formulated into a dilute isopropanol solution and added to the mixing system at the stage after mixing the active material, the binder, and other components (refer to paragraph 0039 and paragraph 0046). Further, it is disclosed that the amount of acetylene glycol or a derivative thereof to be mixed is from 20 to 5,000 ppm in a slurry containing the active material (refer to paragraph 0018).
In addition, Examples in PTL 1 disclose that when an acetylene glycol derivative (having a content in slurry of 150 ppm) is used in combination with a binder (having a polymer content of 3 parts by mass) in which a polymer latex essentially comprising styrene, butadiene, and methyl methacrylate is used, the capacity retention has been improved from 78% to 82% in the charge and discharge cycle test of 30 cycles (refer to paragraph 0042, Example 1, and paragraph 0052, Example 4). However, since the binder and acetylene glycol or a derivative thereof are separately added in the preparation of the slurry, there is a concern that both the components cannot necessarily be sufficiently mixed in the mixing system. In addition, since acetylene glycol or a derivative thereof is formulated into an isopropanol solution and then used, an appearance defect of electrodes resulting from isopropanol easily occurs. In addition to these problems, since the amount of the binder used relative to that of the electrode active material is large, a battery obtained by using this slurry has high internal resistance and is insufficient also in terms of a charge-discharge cycle property.
On the other hand, PTL 2 proposes a method involving using a non-diene-based polymer essentially comprising styrene and an ethylenically unsaturated carboxylic ester as a water-dispersed binder instead of conventional SBR. It is disclosed that when this binder is used, the binding property between active materials and between an active material and a collector is satisfactory, and that a charge-discharge cycle property is improved. In an Example, 90% by mass of the active material and 2% by mass of the binder (in terms of non-volatile matter) are mixed to prepare a slurry (refer to paragraph 0073, Example 1). Similar to the case of PTL 1, the battery manufactured using this slurry has a problem derived from the fact that the amount of the binder used is large, and for example, a problem is that internal resistance increases. Particularly, when a cycle test is performed, a poor cycle property is easily observed because the internal resistance easily increases to cause reduction in voltage. In addition, there was room for improvement in the binding property between active materials.